Integrated class D audio amplifiers have been around for more than 10 years and steadily gained popularity due to numerous advantageous characteristics such as high power conversion efficiency, small dimensions, low heat generation and good sound quality. Bipolar CMOS and DMOS high voltage semiconductor processes are typical candidates for implementation of these integrated class D audio amplifiers which feature large LDMOS devices as active switches of output stages. These LDMOS transistors are isolated high side devices and typically of NMOS type to minimize transistor dimensions for a given output resistance. As the bipolar CMOS and DMOS high voltage semiconductor processes continue to evolve to feature sizes at 180 nm and below, the gate drive voltage required to the LDMOS active switches is approaching a voltage level around 5 V. This gate drive voltage should not be exceeded by the integrated high side gate driver structure to maintain gate integrity because the gate-source voltage of the high side LDMOS transistor should always be limited to a voltage range that fits an oxide voltage range of the LDMOS transistor in question such as the above-mentioned 5 V. This accuracy requirement complicates the provision of an adequate DC supply voltage, i.e. the high side positive supply voltage, to the integrated high side gate driver structure driving a high side LDMOS transistor. Traditionally, the accuracy and stability of the gate-source voltage supplied to the high side LDMOS transistor has been solved by using an external bootstrap capacitor for the DC supply voltage of the gate driver of every high side LDMOS transistor.
However, such external capacitor(s) adds component and assembly costs to the integrated class D audio amplifier to an extent that is unacceptable in numerous types of applications such as high-volume consumer audio systems. To further worsen the situation, a typical Class D audio amplifier may include numerous high side power transistors and associated high side gate driver structures or circuits that each needs an external capacitor for example in H-bridge output stages of multi-level PWM amplifiers. Consequently, it is highly desirable to provide a novel high side gate driver structure and circuit capable of accurately driving a high side LDMOS transistor, and other types of high side power transistors, without any need for an external capacitor to stabilize the high side positive supply voltage for the high side gate driver.
This has been accomplished by the present high side gate driver structure which comprises a novel type of double junction isolated well structure with an extra buried semiconductor layer. The present high side gate driver structure eliminates parasitic well structure to semiconductor substrate capacitance at the high side positive DC supply voltage of the high side gate driver which allows elimination of the above-discussed traditional external bootstrap capacitor.